The present methods of modulating a beam of light include mechanical shutters, electro-optic devices and acousto-optic devices. Mechanical modulators are inherently slow since they require the movement of mechanical parts over distances of the order of millimeters. The electro-optic devices involve means for rotating the polarization state of an incident light beam. This is inherently a high voltage process, on the order of several thousands of volts, which causes complications in the electronics and also results in a slower response.
The acousto-optic modulator operates by deflecting an incident light beam by diffraction from a periodic refractive index variation introduced into an optically transparent material by an acoustic wave. These are inherently small aperture devices because the finite velocity of the acoustic wave requires the light beam to have a small cross section in order for the device to have a fast response time.
The patent to Rabadeau U.S. Pat. No. 3,514,183 describes a switchable total internal reflection light deflector which could be used as a light modulator. This deflector has a piezoelectric crystal which is used to flex a glass plate causing its surface to be brought into intimate contact with a prism. The light undergoes frustrated total internal reflection when the plate and prism are less that .lambda./50 apart, that is, of the order of 100 A. This means they are in substantial contact with one another. The forces of attraction between prism and plate are of such a magnitude that they have to be literally peeled apart. This attraction causes cold welding of the glass surfaces and device failure results after a few million cycles. Since the glass plate is forced into and out of optical contact with the prism, the life of the deflector is also limited by the abrasion of the interface surfaces.